This invention relates to a method and to a device for determining and/or checking the state or the condition of a fluid contained in a hydraulic system such as a hygroscopic brake fluid. More particularly, the invention relates to a method and device for determining the boiling point or a characteristic of the fluid dependent on the boiling point by means of sensor elements. The invention also relates to circuit configurations for implementing the method and for controlling the device.
It is already known to check the state of a hydraulic fluid, namely a hygroscopic brake fluid, by measuring the boiling point of the fluid. Due to unavoidable absorption of water, the boiling point of the fluid can drop in the course of time such that in case of hard braking, the heat induced into the fluid can cause the formation of vapor bubbles which impairs the operativeness of the brake.
Changing brake fluid after one or two years such as is presently recommended is not an optimal solution to the problem since the process of aging, and in particular the absorption of water and accompanying boiling point reduction caused thereby, depends on numerous parameters such as climate, air humidity, mode of operation, and condition of the brake system and thus varies within very wide limits. Further, even a fresh brake fluid does not guarantee that in case of a strong strain on the brakes, such as in case of a prolonged, continuous application, the temperature of the brake fluid will remain below the value where dangerous vapor bubbles can form.
Methods and devices for measuring the boiling temperature of a brake fluid in the workshop or in laboratories are likewise known. According to a German industrial standard, a relatively accurate measuring method is known. However, it can be implemented only by trained staff and with relatively great expenditure of time and expensive equipment.
In the European Patent Specifications Nos. 56 424 and 74 415, measuring methods and measuring probes are described which are immersed in the fluid to be examined and which heat up a small amount of the fluid and ascertain the temperature at the onset of boiling. There are doubts as to whether it is possible to determine the boiling point of a brake fluid with sufficient accuracy in this manner, because, in fact, only the temperature of the pertaining heating element can be determined, and not the boiling temperature of the fluid.
One method for determining the condition and the state of a hydraulic fluid is disclosed in German Patent Application No. P 35 22 774.5 (DE-OS 35 22 774), published Dec. 29, 1986 wherein a sensor element is heated such as to ensure that there comes about a stable cellular convection in a temperature range lying below the boiling temperature of the fluid. The temperature in this phase is then measured either directly or indirectly as a voltage drop across the sensor element which voltage drop is suitable as an indication of the boiling point of the fluid and allows a relatively accurate determination of the boiling temperature.
Likewise, disclosed in the German Published Patent Application (DE-OS) No. 33 17 638 (U.S. Pat. No. 4,566,805) is a device for checking the brake fluid which is stationarily built into the brake system. According to this Published Patent Application, the actual detector can be designed as a component of a bleeder screw.